Recently, the effective utilization of water source is required because of industrial development and population growth. In this point of view, it is very important to reuse waste water such as industrial waste water. The reuse of the waste water can be realized by purifying the waste water, that is, separating other contents from the waste water except the water.
Various methods are known as a method for separating other contents from a liquid. For example, membrane separation, centrifugal separation, activated carbon adsorption, ozone treatment, agglomeration and suspended matter removal with adsorbents can be exemplified. The use of such a method as mentioned above can remove chemical materials such as phosphorous components and nitrogen components contained in water which affect the environment remarkably, and can remove oil and clay dispersed in the water.
Among the separating methods as mentioned above, the membrane separation is one of the most commonly used separating methods, but may be likely to be suffer from the clogging of the fine pores of the membrane in the case of the removal of the oil dispersed in the water, causing the disadvantage that the lifetime of the membrane is shortened. In this point of view, the membrane separation is not appropriate for the removal of the oil from the water in many situations. As a method for removing oil such as heavy oil, therefore, such a removing method as collecting the heavy oil floating on the water surface with an oil boom provided on the water surface, which utilizes the floating property of the heavy oil, adsorbing and recovering the floating heavy oil is employed. Alternatively, such a removing method as laying a hydrophobic material with absorptivity for the heavy oil on the water surface, adsorbing and recovering the floating heavy oil is employed.
Recently, in this point of view, such an attempt as using oil adsorbents is made. Concretely, the oil adsorbents are immersed in the water containing the oil dispersed therein so as to adsorb the oil and then, the oil adsorbents with the adsorbed oil are removed from the water. For example, Reference 1 teaches that oil adsorbents made of respective magnetic particles and respective organic components such as resins covering the corresponding surfaces of the magnetic particles are used so as to adsorb and remove the oil from the water. In this method, however, the dispersion of the oil adsorbents is not excellent so that the oil adsorbent tend to be settled out or floated on the water surface. Therefore, the oil cannot be adsorbed and removed efficiently and effectively by the oil adsorbents.
Moreover, Reference 2 teaches that the oil is adsorbed by the adsorbing polymers as oil adsorbents which include respective hydrophilic blocks and respective hydrophobic blocks, and the adsorbing polymers with the adsorbed oil are removed from the water. In this method, however, the separation between the adsorbing polymers and the water is difficult, causing the spending of one' s labor and more, the adsorbing polymers with the adsorbed oil is softened, causing the deterioration of workability.
On the other hand, Reference 3 teaches that the oil is adsorbed by the magnetized adsorbing particles so that the adsorbing particles with the adsorbed oil are separated by means of magnetic force. For example, the surfaces of the magnetic particles are modified by stearic acid such that the oil in the water is adsorbed and recovered by the magnetic particles. In this method, however, since the surfaces of the magnetic particles are modified by stearic acid or coupling agent of low molecular weight, the thus generated low molecular weight components may contaminate the water adversely.
With all of the removing methods as described in References 1 to 3, since the oil adsorbents are disposed after the adsorption of the oil, the use efficiency of the oil adsorbents is low. Moreover, if the oil adsorbents are not up to standard, the oil adsorbents are disposed as they are. In this point of view, the use efficiency of the oil adsorbents is deteriorated. As a result, a relatively large amount of oil adsorbent is required in order to adsorb and remove the oil to be removed, resulting in the increase in cost relating to the oil removing operation inherently.